On my '37 seat bottoms, I had a few springs in not very good condition. I found some heavy foam and snaked it between the springs and the outer edge of the seat. This made the edge not droop down and really supported the backs of my knees. It now sets like a good bucket seat does, has support around the edge, but soft in the middle.
I suspect you could do the same with the seat back. Just remove the damage springs, and add dense foam up in the spring area. I hunted around for springs also, to many to choose from and to many different spring rates to know what I needed. The foam came from a local craft store so I could set on it and press on it before purchasing it, I knew what I was getting.
I used a vinyl seat cover over the springs as the first layer. This kind-of leveled out the base layer. Burlap I believe was the original base layer, I found it to soft and lumpy, the vinyl took a lot of the lumpiness out. Working at a bus garage, I had access to a lot of damaged school bus seat covers.
Joe
